I can't remember which carrier initially compelled Google to do this, but it was done to save their networks rather than save cost for Google. It may have been Verizon when the HTC Thunderbolt launched. Now all the carriers are on-board.
> All detectable video streaming is optimized for your mobile device so you can watch up to three times more video using the same amount of high-speed data.
e.g., https://www.t-mobile.com/offer/binge-on-streaming-video.html
> All detectable video streaming is optimized for your mobile device so you can watch up to three times more video using the same amount of high-speed data.